In recent years, a leadless BGA (Ball Grid Array) is adopted as one type of component package for a high-density circuit board for mounting electronic components. This is because electronic components can be packaged in a BGA at a density higher than that of a package having terminals disposed on a peripheral portion, such as a QFP (Quad Flat Package) having a lead on the periphery of a semiconductor package, so the use of a BGA makes it easy to form a multipin structure. In addition, electronic components can be collectively packaged in a BGA upon a so-called reflow soldering process. The reflow soldering means herein a process of printing a solder paste (a material obtained by kneading a solder powder and a flux with each other into a paste) on lands of a circuit board for mounting electronic components, and heating the circuit board as the electronic components are mounted in it to melt, for example, the printed solder balls (also called bumps), thereby soldering the electronic components to the circuit board.
Japanese Patent Laid-Open No. 2007-149828 discloses a circuit board for mounting, by a reflow soldering, electronic components in which a plurality of solder balls are two-dimensionally arranged on their surfaces in, for example, a BGA. In this circuit board for mounting electronic components, due to its warpage, failures in connection of the solder balls occur more frequently on the periphery of the BGA than at or near the center of the BGA. Hence, the size of the land of each solder ball on the periphery of the BGA is set larger than that of the land of each solder ball at or near the center of the BGA. In general, as the size of the land of each solder ball increases, the bonding area with the solder increases, so the surface tension (self-alignment effect) of this solder ball upon melting also increases. Hence, the circuit board can withstand external stresses such as warpage of the circuit board.
However, as in the above-mentioned circuit board for mounting electronic components described in Japanese Patent Laid-Open No. 2007-149828, when the size of the land of an interposer of a BGA chip is set differently to that of the land of the circuit board, the following problem is posed. That is, when a BGA chip is mounted on the secondary surface of the circuit board, the following phenomenon may occur, leading to failures in connection of the solder balls.
The phenomenon in which a failure in connection of any of solder balls 307 occurs in a BGA chip 303 on the secondary surface of a circuit board 301 will be explained with reference to FIGS. 3 and 4.
FIG. 3 depicts a sectional view showing the circuit board 301, in which its secondary surface (its upper surface in FIG. 3) is reflow soldered first, and the BGA chip 303 is mounted. To attain a strong solder bond, the size of a land 311 of the circuit board 301 is set larger than that of a land 310 of an interposer 305 of the BGA chip 303, as shown in FIG. 3.
After the secondary surface of the circuit board 301 is thus reflow soldered, the primary surface of the circuit board 301 (its upper surface in FIG. 4) is reflow soldered next. Note that as shown in FIG. 4, during the reflow soldering process of the primary surface of the circuit board 301, the BGA chip 303 is present below the circuit board 301, so a force acts on the BGA chip 303 downwards in a direction indicated by an arrow 401 by the action of gravity. Also, as the circuit board 301 is heated by reflow soldering process of its primary surface, the solder balls 307 on the secondary surface of the circuit board 301 melt. At this time, since the surface tension of the solder is proportional to the size of the land, (Surface Tension of Interposer)<(Surface Tension of Circuit Board), and this means that the solder ball 307 is pulled toward the land 311 of the circuit board 301. As a result, the solder on the smaller land, that is, the solder on the land of the interposer 305 is cut, leading to a failure in connection.